Previous month's last business day

Please note that if you have holidays in that month, my above formula is ignoring them. List your holidays (as dates), say, on A1:A10, and then you can add ",A1:A10" (without quotes) to the formula above:
=WORKDAY(EOMONTH(TODAY(),-1)-7,5,A1:A10)

EDIT: Oh, I almost forgot, please note that this formula is volatile. Next month in December, the formula will return November's last working day instead of October's. This seem you already know - you need to activate the analysis pack in the tools menu, and all those who will use the file need to do the same.